Steam plant with pressure-fired boiler

ABSTRACT

A steam power plant providing steam to drive a steam turbine coupled to an electrical generator includes a pressure-fired boiler the fuel flow rate of which is controlled by a livesteam regulator and a pressure-charging set consisting of turbocompressor for providing combustion air and which is driven by a gas turbine powered by the exhaust gas from the boiler and a starter motor, all on the same shaft. The speed of the pressurecharging set is regulated by varying the throughput of the gas turbine and/or the temperature of the gas turbine inlet in such manner that it neither produces surplus power nor requires power from the outside in accordance with variations in the steam power demand from the boiler as the load on the steam turbine changes. The starter motor for the pressure charging set is disconnected after starting and, at full load on the plant, the charging pressure for the combustion air is at least 9 bar, and the exhaust gas at the gas turbine outlet is no higher than 165*.

United States Patent Pfenninger STEAM PLANT WITH PRESSURE-FIRED BOILER[75] Inventor: Hans Pfenninger, Baden,

Switzerland [73] Assignee: BBC Brown Boveri & Company Limited, Baden,Switzerland [22] Filed: Nov. 27, 1973 [21] App]. No.: 419,276

[30] Foreign Application Priority Data Dec. 1, 1972 Switzerland 17481/72[52] US. Cl. 60/39.l8 B [51] Int. Cl. F02c 9/02; FOlk 23/00 [58] Fieldof Search 60/105, 39.18 B

[56] References Cited UNITED STATES PATENTS 2,911,789 11/1959 Baker60/39.]8 B

3,203,175 8/1965 Michalicka e 60/39.]8 B 3,232,052 2/1966 Ricard60/39.]8 B

Primary Examiner-Martin P. Schwadron Assistant Examiner-Allen M.Ostrager Attorney, Agent, or FirmPierce, Scheffler & Parker [57]ABSTRACT A steam power plant providing steam to drive a steam turbinecoupled to an electrical generator includes a pressure-fired boiler thefuel flow rate of which is controlled by a livesteam regulator and apressurecharging set consisting of turbocompressor for providingcombustion air and which is driven by a gas turbine powered by theexhaust gas from the boiler and a starter motor, all on the same shaft.The speed of the pressure-charging set is regulated by varying thethroughput of the gas turbine and/or the temperature of the gas turbineinlet in such manner that it neither produces surplus power nor requirespower from the outside in accordance with variations in the steam powerdemand from the boiler as the load on the steam turbine changes. Thestarter motor for the pressure charging set is disconnected afterstarting and, at full load on the plant, the charging pressure for thecombustion air is at least 9 bar, and the exhaust gas at the gas turbineoutlet is no higher than 165.

3 Claims, 1 Drawing Figure STEAM PLANT WITH PRESSURE-FIRED BOILER Thisinvention concerns a steam power plant with pressure-fired boiler thefuel flow rate of which is controlled by a livesteam regulator, andwhich incorporates a pressure-charging set comprising a turbocompressor,a gas turbine connected after the boiler and a starter motor, the speedor" the set being variable in order to match the required flow rate ofcombustion air to the adjusted fuel flow rate.

Pressure-fired boilers for steam power plant have long been known underthe name of Velox boilers. The pressure-charging group is usuallycontrolled by means of a Ward-Leonard set which is permanently coupledto the pressure-charging group. The fuel flow rate is controlled interms of steam consumption which at the same time influences theWard-Leonard set in known manner and thus regulates the speed of thepressure-charging set, or in other words matches the required air flowrate to the fuel flow rate. At approximately three-quarters load thepower output of the gas turbine just covers the power requirement of thecompressor. At lower partial load the pressure-charging set consumesadditional electrical power, while at higher part load, full load andoverload surplus power is fed into the network. The Ward Leonardconfiguration allows speed variation within broad limits, but has thedisadvantage that the set runs continuously. It is also costly.

Steam boilers of this kind at full load usually have a charging pressureof only 2 3 bar and high gas velocities, and in consequence the gasturbine exhaust temperature is relatively high, thus necessitating asubsequent heating surface which is no longer at the compressordischarge pressure and is therefore very large and expensive. Since forpractical reasons only one economiser can be considered for thesubsequent heating surface, the feed water temperature must be reducedby bleed steam, which impairs the thermal efficiency of the process. Inaddition, the condenser must be made larger.

The impaired efficiency of the process due to the lower rise in feedwater temperature can be counteracted in known manner by employing acombined process in which the gas turbine set, which at the same timeserves to pressure charge the boiler, is operated at the maximumpermissible temperature at the gas turbine inlet. This method has theeffect of reducing the steam output because the hot gases are cooled toa lesser extent in the boiler, but on the other hand the gas turbineproduces useful power by way of a generator. With a combined process thelosses due to the smaller rise in feed water temperature can just beovercome, but at the same time the following disadvantages must be takeninto account:

Because of the corrosion risk, the high temperature at the gas turbineinlet means that only a very clean,

therefore be large. The heat-exchange area of this nonpressurised heatexchanger is a multiple of those in the pressure-charged boiler. Thelarge volume of the heat exchanger also sharply increases the spacerequired and the cost of the foundation.

The power produced must be divided between two generators, one for thegas turbine and one for the steam turbine, thus increasing the plantcosts.

Since the gas turbine has a generator, its speed is constant and at partload the air rate cannot be reduced, or only at great expense, resultingin high exhaust losses. This can in fact be avoided by using a separatepower turbine, but again the plant cost would be much higher.

So that no evaporation takes place in the economiser at part load thetemperature at the gas turbine inlet must be lowered, thus impairing thethermal efficiency.

The object of the invention is to create a structurally simple steampower plant of the kind mentioned above with good overall efficiency,such that on the gas side no additional heat exchanger is required afterthe pressure-fired boiler and yet the stack losses remain within thelimits customary with such plant.

This object is achieved in that the speed of the pressure-charging setcan be regulated by varying the throughput of the gas turbine and/or thetemperature at the gas turbine inlet so that it neither produces surpluspower nor requires power from outside, the starter motor can bedisconnected after starting, at full load the charging pressure is atleast 9 bar and the temperature of the gas turbine exhaust is no higherthan C.

With this simple, dependable and very economical plant, speed isregulated by means inherent to the boiler, no intervention from outsideis necessary and no external power is required.

The speed, and hence also the air flow rate, is continuously matched tothe fuel flow rate so that excess air in the boiler remains practicallyconstant under all operating conditions and is restored immediatelyafter load changes. The high pressure ratio of the pressurecharging setmeans that the temperature of the compressed combustion air is high,allowing even heavy oil to be burned without difficulty and without theneed for additional preheating of the air. Because of the high pressureratio the dimensions of the steam generator can be small, allowing it tobe transported in the assembled condition (packaged unit), and also theexhaust temperatures are low without the pressure-charging set sendingout power so that no feed water heating by the exhaust gases isnecessary and the thermal process can be optimised by heating the feedwater with bleed steam.

The schematic, simplified drawing shows an example of the invention. Thesteam power plant consists essentially of a high-pressure turbine 1 anda low-pressure turbine 2, which together drive the electric generator 3,of a condenser 4, condensate pump 32, feed water heaters 5, heated bybleed steam, and steam boiler 6 which includes a vaporizer section a andan economizer section 31 for pre-heating the returning feed water. Theboiler is pressurised, i.e. brought to the required gas-side pressure,by means of the pressure-charging set which basically comprises aturbocompressor 7, gas turbine 8, starter motor 9' and hydrauliccoupling 10. All components of the pressure-charging set are mounted onthe same shaft.

Gas turbine 8 is driven by the hot combustion gases discharged from thesteam boiler 6, the temperature of the combustion gas delivered to theturbine inlet being controlled by a valve 29 which controls the amountof combustion gas permitted to by-pass the economizer section 31 througha by-pass duct 30. That is to say, when valve 29 is open part of the hotcombustion gas exiting from the boiler by-passes the economizer section31 by direct flow through duct 30 and the remaining part of thecombustion gases pass through the economizer section 31 and arepartially cooled off. The two partial gas flows mix and are thusdelivered to the gas inlet of turbine 8. When valve 29 is closed, all ofthe combustion gas is forced to. pass through the economizer section 31and cooled off with the result that the temperature in front of turbine8 will be correspondingly lower. A rise in temperature of the combustiongas exiting from the steam generator results in a higher turbine speedand hence a higher speed for the compressor 7 driven by it and a greateramount of combustion air delivered to the steam generator. Conversely adecrease in temperature of the combustion gas exiting from the steamgenerator results in a decrease in speed of the charging group 7, 8 andhence a decrease in combustion air from compressor 7.

The control system of the plant includes the following individualcomponents of interest in the present context: the steam pressure orflow rate in the live steam line 11 influence the live steam regulator12 which controls drain 13 on pressure-oil line 14 of the v primarysystem, this line being fed at 15 by way of throttle 16. Pressure-oilline 14 is connected at one end to servo 17, which regulates the fuelsupply (not shown) via fuel nozzles 18 in the boiler, and at the otherend to servo 19 which moves the cylinder 20 of centrifugal governor 21of the pressure-charging set, whereupon the oil pressure in thepressure-oil line 23 of the secondary system is varied by means of drain22. Pressure-oil line 23 is fed at 24 by way of throttle 25 and leads tovalve 26 on the bypass line 27 round gas turbine 8 and to the servo 28which actuates valve 29, which in turn influences bypass 30 for theeconomizer section 31. The hydraulic control system can also be replacedby an electrical control system with the same functions.

"The interaction between the individual parts and the method ofcontrolling the plant are described below.

At full load the turbocompressor 1 compresses the a combustion air to atleast 9 bar, thus heating it to approx. 330C, which facilitates theburning of heavy oil. The boiler 6 is so designed that the gastemperature at the outlet is approx. 430C, which is also the inlettemperature to the gas turbine 8. In association with the high pressureratio, this results in an exhaust gas temperature after the gas turbine,and hence a stack temperature, of only 150C. The exhaust loss can thusbe kept very low and a bulky extra heating surface avoided. Theturbocompressor has no form of cooling and thus the feed water does nothave to be heated either by the compressed air or by the exhaust gas,which allows optimum heating by means of bleed steam.

The temperature at the gas turbine inlet is kept down, by the controlsystem described below, to a value which just allows the gas turbine todrive the turbocompressor of the pressure-charging set. As a result,with the exception of the bearing and radiation losses, the entire heatof expansion of the gas turbine passes to the compressor so that thepressure-charging set has the effect of the exhaust-gas-heated airheater required with known kinds of plant. At the same time, however, itgenerates a high pressure and thus reduces the boiler heating surfacearea to a fraction of that in a nonpressurised boiler.

In order to be able to reduce the speed of the pressurecharging set atpartial load which has the advantage that the combustion air rate can atany time be matched to the boiler load or fuel rate starter motor 9 isdisconnected from the pressure-charging set by disengaging coupling 10,and stopped as soon as the pressure-charging set, after the combustorhas ignited, reaches the speed required for equilibrium betweencompressor power and gas turbine power. This is then subject to theaction of the control system. In this way the combustion air flow ratecan be reduced to some 55 percent at a boiler load of approx. 50percent, so that the excess air factor in the boiler can be keptpractically constant between full and half load. This is important withrespect to clean combustion and correspondingly less atmosphericpollution.

If the live steam pressure falls, for example, because the steamturboset requires a higher steam rate owing to a prolonged increase inoutput, the live steam regulator 12 closes drain 13 on pressure-oil line14. The pressure in the primary system then rises and, by way of servo17, opens the fuel nozzles 18 in the boiler. At the same time servo 19moves the cylinder 20 of centrifugal governor 21 so that drain 22 isclosed, thus raising the oil pressure in pressure-oil line 23 of theindependent secondary system. The result of this is that valve 26 inbypass line 27 round the gas turbine 8 closes, and servo 28 causes valve29, and hence bypass 30 on the gas exit side of the boiler, to open. Inconsequence, the flow rate through the gas turbine increases and thetemperature at the gas turbine inlet is raised; thus the speed of thepressure-charging set, or the combustion air flow rate, is adapted tothe new, higher fuel rate.

In the event of temporarily rising live steam pressure, i.e. fallingload, the control process takes place in a similar manner, but in theopposite sense.

From the standpoint of efficiency it is of advantage not to actuatevalves 26 and 29 simultaneously, but in the case of increasing demandfor combustion air first to close bypass 27 round the gas turbine 8 andonly then to open bypass 30 on the gas exit side of the boiler, and inthe case of decreasing combustion air demand first to close 30 on thegas exit side of the boiler and then to open bypass 27 round the gasturbine 8. In this event the two control procedures can overlap to somedegree, but it is also possible to leave a small neutral zone betweenthem. These various possibilities can easily be achieved by suitablyselecting the spring forces of valve 26 and servo 28, in which case thespring of the servo must be the stronger.

With this arrangement it may be most efficient to select the springforces so that at full load both bypasses are closed. If the actual loadexceeds the full load, then bypass 30 on the gas exit side of boiler 6opens at least temporarily in order to provide the combustion air flowneeded for the higher fuel flow rate as quickly as possible, and closesagain when the actual load decreases. If the actual load is less thanfull load, bypass 27 round gas turbine 8 opens temporarily in order tomatch the speed of the pressure-charging set as quickly as possible tothe lower fuel rate. In this way the speed or combustion air rate can bematched as quickly as possible to the fuel rate.

These sequences of events can be accelerated by using proportionalcontrollers which respond to the rate of change, instead of ordinarycontrollers.

It is also possible for the pressure-charging set to alter its speedwith the live steam regulator 12 at a constant setting, i.e., atconstant load. This happens if the outside air temperature fluctuatesvery widely, for example. in this case, too, the control system performsits function in full.

lf, under otherwise constant conditions the outside air temperaturerises, the weight of air discharged by the compressor 7 decreases,whereupon the pressurecharging set is no longer in equilibrium, and itsspeed begins to fall. Oil drain 22 is closed by centrifugal governor 21and the pressure in oil line 23 of the secondary system starts to rise.As a result, bypass valve 26 is closed and/or valve 29 is opened,whereupon the flow rate through the turbine and/or the temperature atthe gas turbine inlet rises until the desired speed is regained. 'z

The entire control system can also be so designed that the pressure inpressure-oil lines 14 and 23 decreases when the live steam pressurefalls, but the effects remain the same.

I claim:

l. A combined steam and gas turbine plant comprising a pressure-firedboiler providing steam for driving a steam turbine coupled to a powerconsumer such as an electrical generator, means including a condenser atthe discharge side of said steam turbine for converting the dischargedsteam into feed water and a return line for the feed water to the inletside of said boiler for recycling, a heat exchanger for pre-heating thereturned feed water by heat exchange with combustion gas at the gasdischarge side of said boiler, a by-pass for the combustion gas passingthrough said heat exchanger, valve means for controlling said by-pass,the respective portions of the combustion gas put through said by-passand through said heat exchanger being combined at the outlet thereof anddelivered to the inlet of a gas turbine at a temperature variable inaccordance with the portion of the gas put through said by-pass andconstituting the sole motive fluid for driving said gas turbine, alivesteam regulator responsive to a change in load on the power plantfor correspondingly regulating the fuel supply to said boiler and forregulating said valve means which controls the by-pass to said feedwater heat exchanger thereby to increase or decrease, respectively thetemperature of the discharged combustion gas and thereby increase ordecrease, respectively the speed of the gas turbine and air compressorcoupled thereto in dependence upon the sense of the change in load onthe power plant so as to maintain a match between the required powerinput to said gas turbine and the required pressurized air output fromsaid compressor.

2. A power plant as defined in claim 1 and which further includes'anadditional by-pass provided between the inlet and outlet sides of saidgas turbine, and valve means for controlling said additional by-pass andwhich are likewise regulated by said live-steam regulator in such senseas to move said valve means to a more closed or open positionrespectively in response to an increase or decrease respectively in loadon the power plant, said valve means for controlling the amount ofcombustion gas put through the by-pass for the heat exchanger beingmoved to a more open position followed by a movement of said valve meansfor controlling the amount of combustion gas by-passed between the inletand outlet sides of said gas turbine to a more closed position in theevent of an increase in load on the power plant, and said valve meansfor controlling the amount of combustion gas put through the by-pass forthe heat exchanger being moved to a more closed position followed bymovement of said valve means for controlling the amount of combustiongas by-passed between the inlet and outlet sides of said gas turbine toa more open position in the event of a decrease in load on the powerplant.

3. A power plant as defined in claim 2 wherein both of said valve meansfor controlling said by-passes are closed when said power plant isoperating at its full load rating.

1. A combined steam and gas turbine plant comprising a pressurefiredboiler providing steam for driving a steam turbine coupled to a powerconsumer such as an electrical generator, means including a condenser atthe discharge side of said steam turbine for converting the dischargedsteam into feed water and a return line for the feed water to the inletside of said boiler for recycling, a heat exchanger for pre-heating thereturned feed water by heat exchange with combustion gas at the gasdischarge side of said boiler, a by-pass for the combustion gas passingthrough said heat exchanger, valve means for controlling said by-pass,the respective portions of the combustion gas put through said by-passand through said heat exchanger being combined at the outlet thereof anddelivered to the inlet of a gas turbine at a temperature variable inaccordance with the portion of the gas put through said by-pass andconstituting the sole motive fluid for driving said gas turbine, alive-steam regulator responsive to a change in load on the power plantfor correspondingly regulating the fuel supply to said boiler and forregulating said valve means which controls the by-pass to said feedwater heat exchanger thereby to increase or decrease, respectively thetemperature of the discharged combustion gas and thereby increase ordecrease, respectively the speed of the gas turbine and air compressorcoupled thereto in dependence upon the sense of the change in load onthe power plant so as to maintain a match between the required powerinput to said gas turbine and the required pressurized air output fromsaid compressor.
 2. A power plant as defined in claim 1 and whichfurther includes an additional by-pass provided between the inlet andoutlet sides of said gas turbine, and valve means for controlling saidadditional by-pass and which are likewise regulated by said live-steamregulator in such sense as to move said valve means to a more closed oropen position respectively in response to an increase or decreaserespectively in load on the power plant, said valve means forcontrolling the amount of combustion gas put through the by-pass for theheat exchanger being moved to a more open position followed by amovement of said valve means for controlling the amount of combustiongas by-passed between the inlet and outlet sides of said gas turbine toa more closed position in the event of an increase in load on the powerplant, and said valve means for controlling the amount of combustion gasput through the by-pass for the heat exchanger being moved to a moreclosed position followed by movement of said valve means for controllingthe amount of combustion gas by-passed between the inlet and outletsides of said gas turbine to a more open position in the event of adecrease in load on the power plant.
 3. A power plant as defined inclaim 2 wherein both of said valve means for controlling said by-passesare closed when said power plant is operating at its full load rating.